One type of film-covered electric device is a film packed battery. Conventionally, there is known a film packed battery which includes a battery element sandwiched by casing films from both sides in its thickness direction, with the opposing casing films bonded to each other along the periphery of the battery element, to hermetically seal (hereinafter simply called “seal” as well) the battery element together with an electrolytic solution. A laminate film which has a laminate of a metal layer and a thermo-fusing resin layer is generally used for the casing films, and the casing films are bonded to each other by thermally fusing the thermo-fusing resin layers to each other.
Incidentally, when a voltage beyond a rated range is applied to a battery in use, gas species can generate within the battery due to electric decomposition of an electrolytic solvent. Further, when a battery is used at high temperatures beyond a rated range, materials which cause gas species are also generated due to decomposition of electrolyte salt and the like. Basically, it is ideal to use a battery within a rated range to prevent the generation of gases. However, if a control circuit of a battery does not normally operate for some reason to apply an abnormal voltage, or if the temperature abnormally rises around the battery for some reason, a large amount of gases can generate within the battery in some cases.
The generation of gases within a battery results in an increased pressure within the battery. In order to prevent a battery from bursting due to an extremely increased inner pressure, many of batteries which use a highly rigid packing material such as a metal can and a thick resin molding have a pressure safety valve for letting gases escape to the outside of the battery when the inner pressure of the battery increases. However, in film packed batteries which employ a film for a packing material, it is structurally difficult to provide a pressure safety valve. If the inner pressure extremely increases in a film packed battery, a space which contains a battery element swells, and the film is eventually burst, causing gases to blow out from the burst site. However, it is impossible to identify where the burst occurs. Therefore, the burst can adversely affect surrounding devices and members depending on the location at which the film is burst.
It is often the case that batteries are also used as energy sources for vehicles such as a car and a variety of machines, by way of example, or their-electric parts. Since gases generated from the interior of batteries can contain a combustible material and a corrosive material, importance should be placed on how to handle gases which blow out. JP2003-45380A (Patent Document 1) describes a car-mounted battery which includes a gas flow path within a battery box, where the gas flow path is coupled to an exhaust pipe through which gases generated within the battery box are discharged to the outside of a car.
JP2002-324526A (Patent Document 2) in turn describes a film packed battery which comprises a battery element sealed in layers by aluminum foil and covered with a plurality of covering materials, made of film, each having an opening. Each covering material is thermo-contractile, and covers the battery element such that their openings do not overlap with each other. According to the battery described in Patent Document 2, when gases are generated from the battery element, the aluminum foil breaks, causing the gases to blow out therefrom. The blowing gases sequentially pass through the openings of the respective covering materials from the inside to the outside, and are discharged to the outside of the battery. When the battery is heated, the respective covering materials contract such that the opening of the covering material on the inner side is sealed by the covering material on the outer side. In this way, external air including oxygen is prevented from flowing into the inside of the battery, thus preventing firing and the like of the battery.
Around a battery, there are often elements, such as terminals of the battery, a peripheral circuit connected to the terminals, contacts of the battery, and the like which, when applied with an electrolytic solution, give rise to troubles such as corrosion, malfunction and the like. Gases which blow out of the battery can contain a mist of electrolytic solution, so that in order to prevent troubles due to the blow-out of the gases, the gasses are preferably guided to and discharged from a location spaced away from those elements which can give rise to troubles, using a pipe, as described in Patent Document 1. In this event, the pipe must be hermetically connected to the packing material. As in Patent Document 1, in a car-mounted battery, the packing material is generally a highly strong member which itself has a sufficient thickness and rigidity. For such a member, it is easy to hermetically connect a pile thereto by employing an appropriate fitting structure, or adding a sealing member such as an O-ring.
However, a structure generally employed for film packed batteries to discharge internally generated gases from a particular site involves forming an area which has a lower bonding strength than the remaining area in a portion of a bonding region of a film which is a packing material, and discharging the gas from that area. Conventional film packed batteries give rise to the aforementioned troubles because there has only been proposed a structure for releasing gases immediately adjacent to the batteries. Thus, it is contemplated to connect a pipe to a film packed battery for guiding gases. However, a simply connected pipe results in difficulties in ensuring the sealability with a film because the pipe is connected to the film at a position from which gases are discharged, i.e., in a region which presents a low bonding strength. As a result, depending on a stress applied to the joint of the film with the pipe, the film peels off at the joint with the pipe to let gases leak, thus possibly failing to guide the gases to a desired position. On the other hand, in the film packed battery described in Patent Document 2, openings of the respective covering materials which cover the battery element in layers are simply shifted, without the need for forming an area with a lower bonding strength, so that a pipe can be hermetically connected to the outermost opening. However, since the film packed battery described in Patent Document 2 covers the battery element in layers, not only the amount of used covering materials extremely increases, as compared with ordinary film packed batteries, but also covering steps are also largely increased.
Such problems are common to film-covered electric devices which have electric device elements that can generate gases, sealed by casing films, not limited to the film packed battery.